Implantable cardioverter defibrillators may be used to deliver high energy cardioversion or defibrillation shocks to a patient's heart when atrial or ventricular fibrillation is detected. Cardioversion shocks are typically delivered in synchrony with a detected R-wave when fibrillation detection criteria are met. Defibrillation shocks are typically delivered when fibrillation criteria are met, and the R-wave cannot be discerned from signals sensed by the ICD.
Currently, ICDs use endocardial or epicardial leads which extend from the ICD housing through the venous system to the heart. Electrodes positioned in or adjacent to the heart by the leads are used for pacing and sensing functions. Cardioversion and defibrillation shocks (e.g., anti-tachyarrhythmia shocks) are generally applied between a coil electrode carried by one of the leads and the ICD housing, which acts as an active can electrode.
In addition, or as an alternative to cardioversion and defibrillation shocks, the ICD or an implantable artificial pacemaker may provide cardiac pacing therapy to the heart when the natural pacemaker and/or conduction system of the heart fails to provide synchronized atrial and ventricular contractions at rates and intervals sufficient to sustain healthy patient function. Such antibradycardial pacing may provide relief from symptoms, or even life support, for a patient. Cardiac pacing may also provide electrical overdrive stimulation to suppress or convert tachyarrhythmias, again supplying relief from symptoms and preventing or terminating arrhythmias that could lead to sudden cardiac death.
Cardiac pacing by conventional pacemakers and/or ICDs is usually provided by a pulse generator implanted subcutaneously or sub-muscularly in or near a pectoral region of a patient. The generator typically connects to the proximal end of one or more implanted leads, the distal end of which contains one or more electrodes for positioning adjacent to the inside or outside wall of a cardiac chamber. Each of the leads may be secured near or against the cardiac tissue to provide sufficient transmission of electrical energy to the cardiac tissue in order to capture the heart.